An image sensor generally comprises a zone of useful pixels, i.e. a zone of pixels that are intended to be effectively illuminated, and electronics for controlling these pixels. The pixels and the control electronics are produced in a semiconductor substrate, for example a substrate made of silicon. Image sensors, or “imagers” may be front-illuminated or back-illuminated.
In front-illuminated image sensors, the pixels are illuminated via the front side of the substrate, i.e. the side bearing in particular the interconnect part (commonly referred to as the back end of line (BEOL) part by those skilled in the art).
In contrast, in a back-illuminated image sensor, the pixels are illuminated via the back side of the substrate, i.e. the side opposite the front side.
Moreover, the pixels are generally bounded by deep trench isolations (DTIs) the aim of which is to ensure electrical and optical isolation between the pixels so as to decrease or even prevent crosstalk between adjacent pixels. It will be recalled here that crosstalk is a parasitic signal that originates from a neighboring pixel and that interferes with the signal from the pixel in question. Crosstalk may have an optical and electrical component.
An article by Kitamura et al. entitled “Suppression of Crosstalk by Using Backside Deep Trench Isolation for 1.12 μm Backside Illuminated CMOS Image Sensor”, IEEE 2012 (incorporated by reference), demonstrated that using capacitive deep trench isolations allowed crosstalk to be significantly decreased in back-illuminated image sensors.
In this article, the capacitive deep trench isolations were filled with metal by deposition then chemical mechanical planarization (CMP) so that the metal/dielectric/silicon structure behaved as a capacitor allowing holes to be accumulated around the trench by applying a negative biasing voltage to the filling metal of the trench, thereby decreasing degradation due to dark current.
This being so, this article by Kitamura et al. does not mention the means in the image sensor allowing each of the capacitive trenches to be electrically connected to the biasing voltage. However, this aspect is of importance when designing the image sensor, especially because of the presence of control logic around the zone of pixels.